Quantifying the collisionless nature of dark matter and galaxies in A1689

TL;DR

This study investigates the dynamical differences between galaxies and dark matter in cluster A1689, revealing that within 0.3 times the virial radius, galaxy kinematics are significantly affected by collisional processes unlike dark matter.

Contribution

The paper introduces a method to compare galaxy and dark matter kinematics in a galaxy cluster, highlighting the impact of collisional effects on galaxy dynamics at small radii.

Findings

Galaxies and dark matter exhibit different kinematic behaviors within 0.3r_vir.

Galaxy density profiles flatten compared to dark matter at small radii.

Collisional effects significantly influence galaxy dynamics near the cluster center.

Abstract

We use extensive measurements of the cluster A1689 to assess the expected similarity in the dynamics of galaxies and dark matter (DM) in their motion as collisionless `particles' in the cluster gravitational potential. To do so we derive the radial profile of the specific kinetic energy of the cluster galaxies from the Jeans equation and observational data. Assuming that the specific kinetic energies of galaxies and DM are roughly equal, we obtain the mean value of the DM velocity anisotropy parameter, and the DM density profile. Since this deduced profile has a scale radius that is higher than inferred from lensing observations, we tested the validity of the assumption by repeating the analysis using results of simulations for the profile of the DM velocity anisotropy. Results of both analyses indicate a significant difference between the kinematics of galaxies and DM within $r \lesssim 0.3r_{\rm vir}$. This finding is reflected also in the shape of the galaxy number density profile, which flattens markedly with respect to the steadily rising DM profile at small radii. Thus, $r \sim 0.3r_{\rm vir}$ seems to be a transition region interior to which collisional effects significantly modify the dynamical properties of the galaxy population with respect to those of DM in A1689